Synthesis, structure, and thermal properties of soluble hydrazinium germanium(IV) and tin(IV) selenide salts

Article abstract of DOI:10.1021/ic048276l

The crystal structures of two hydrazinium-based germanium(IV) and tin(IV) selenide salts are determined. (N₂H₅)₄-Ge ₂Se₆ (1) [l4₁ cd, a = 12.708(1) A, c = 21.955(2) A, Z = 8] and (N₂H₄)₃(N ₂H₅)₄Sn₂Se₆ (2) [P1, a = 6.6475(6) A, b = 9.5474(9) A, c = 9.8830(10) A, α = 94.110(2)°, β= 99.429(2)°, γ = 104.141(2)°, Z= 1] each consist of anionic dimers of edge-sharing metal selenide tetrahedra, M ₂Se₆^4- (M = Ge or Sn), separated by hydrazinium cations and, for 2, additional neutral hydrazine molecules. Substantial hydrogen bonding exists among the hydrazine/hydrazinium molecules as well as between the hydrazinium cations and the selenide anions. Whereas the previously reported tin(IV) sulfide system, (N₂H₅)₄Sn ₂S₆, decomposes cleanly to microcrystalline SnS ₂ when heated to 200°C in an inert atmosphere, higher temperatures (>300°C) are required to dissociate selenium from 1 and 2 for the analogous preparations of single-phase metal selenides. The metal chalcogenide salts are highly soluble in hydrazine, as well as in a variety of amines and DMSO, highlighting the potential usefulness of these compounds as precursors for the solution deposition of the corresponding metal chalcogenide films.

Full text of DOI:10.1021/ic048276l

Inorg. Chem. 2005, 44, 3755−3761
Synthesis, Structure, and Thermal Properties of Soluble Hydrazinium
Germanium(IV) and Tin(IV) Selenide Salts
David B. Mitzi
IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598
Received December 8, 2004
The crystal structures of two hydrazinium-based germanium(IV) and tin(IV) selenide salts are determined. (N2H5)4-
Ge2Se6 (1) [I41cd, a 12.708(1) Å, c 21.955(2) Å, Z 8] and (N2H4)3(N2H5)4Sn2Se6 (2) [P1, a 6.6475(6)
Å, b 9.5474(9) Å, c 9.8830(10) Å, R ) 94.110(2) 1] each consist
of anionic dimers of edge-sharing metal selenide tetrahedra, M2Se6 (M Ge or Sn), separated by hydrazinium
)
)
)
h
)
)
)
°
,
â ) 99.429(2)
°
,
)
γ ) 104.141(2)
°
, Z
)
4-
cations and, for 2, additional neutral hydrazine molecules. Substantial hydrogen bonding exists among the hydrazine/
hydrazinium molecules as well as between the hydrazinium cations and the selenide anions. Whereas the previously
reported tin(IV) sulfide system, (N2H5)4Sn2S6, decomposes cleanly to microcrystalline SnS2 when heated to 200
°C
in an inert atmosphere, higher temperatures (>300 C) are required to dissociate selenium from 1 and 2 for the
°
analogous preparations of single-phase metal selenides. The metal chalcogenide salts are highly soluble in hydrazine,
as well as in a variety of amines and DMSO, highlighting the potential usefulness of these compounds as precursors
for the solution deposition of the corresponding metal chalcogenide films.
Introduction
antimony-germanium-telluride phase-change materials,
which currently dominate the CD-RW and DVD markets.16
Most electronic applications of metal chalcogenides and
other semiconductors depend on the ability to deposit
continuous thin films with a well-defined thickness. Common
deposition techniques for the chalcogenides routinely require
relatively high deposition temperatures or vacuum conditions.
Vapor-phase techniques involving vacuum processes, such
as evaporation or sputtering,17,18 and chemical vapor deposi-
tion19,20 are commonly employed to deposit high-quality films
for device applications. Although broadly applicable across
the chalcogenide family, these techniques are not readily
amenable to large area, low-cost deposition. In contrast,
solution-based film deposition is particularly desirable
The diverse structural chemistry and range of electronic
properties exhibited by metal chalcogenides lead to important
opportunities for these materials in contemporary electronic
device research. Some of the earliest solar cells1 and thin
film field-effect transistors (TFTs)2 were, in fact, based on
metal chalcogenide active layers. More recent efforts on
metal chalcogenides have focused on improved transistors,3-7
solar cells,8,9 and thermoelectric10,11 and memory materials.12-16
Commercial applications include optical disks based on
(1) Reynolds, D. C.; Leies, G.; Antes, L. L.; Marburger, R. E. Phys. ReV.
1954, 96, 533.
(2) Weimer, P. K. Proc. IRE 1962, 50, 1462.
(3) Gan, F. Y.; Shih, I. IEEE Trans. Electron DeVices 2002, 49, 15.
(4) Meth, J. S.; Zane, S. G.; Sharp, K. G.; Agrawal, S. Thin Solid Films
2003, 444, 227.
(5) Ridley, B. A.; Nivi, B.; Jacobson, J. M. Science 1999, 286, 746.
(6) Duan, X.; Niu, C.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.;
Goldman, J. L. Nature 2003, 425, 274.
(14) Friedrich, I.; Weidenhof, V.; Njoroge, W.; Franz, P.; Wuttig, M. J.
Appl. Phys. 2000, 87, 4130.
(15) Jongenelis, A. P. J. M.; Coombs, J. H.; van Es-Spiekman, W.; Jacobs,
B. A. J. J. Appl. Phys. 1996, 79, 8349.
(7) Mitzi, D. B.; Kosbar, L. L.; Murray, C. E.; Copel, M.; Afzali, A. Nature
2004, 428, 299.
(8) Britt, J.; Ferekides, C. Appl. Phys. Lett. 1993, 62, 2851.
(9) Contreras, M. A.; Egaas, B.; Ramanathan, K.; Hiltner, J.; Swartzlander,
A.; Hasoon, F.; Noufi, R. Prog. PhotoVolt. Res. Appl. 1999, 7, 311.
(10) Kim, Y.; DiVenere, A.; Wong, G. K. L.; Ketterson, J. B.; Cho, S.;
Meyer, J. R. J. Appl. Phys. 2002, 91, 715.
(16) (a) Ohta, T.; Nishiuchi, K.; Narumi, K.; Kitaoka, Y.; Ishibashi, H.;
Yamada, N.; Kozaki, T. Jpn. J. Appl. Phys., Part 1 2000, 39, 770. (b)
Ohta, T.; Yoshioka, K.; Isomura, H.; Akiyama, T.; Imanaka, R. Proc.
SPIE-Int. Soc. Opt. Eng. 1995, 2514, 302.
(17) Van Calster, A.; Vervaet, A.; De Rycke, I.; De Baets, J. J. Cryst.
Growth 1988, 86, 924.
(18) Chaiken, A.; Nauka, K.; Gibson, G. A.; Lee, H.; Yang, C. C.; Wu, J.;
Ager, J. W.; Yu, K. M.; Walukiewicz, W. J. Appl. Phys. 2003, 94,
2390.
(19) Parkin, I. P.; Price, L. S.; Hibbert, T. G.; Molloy, K. C. J. Mater.
Chem. 2001, 11, 1486.
(11) Sabo, Y. P. J. Thermoelectr. 2001, 4, 58.
(12) Kyratsi, T.; Chrissafis, K.; Wachter, J.; Paraskevopoulos, K. M.;
Kanatzidis, M. G. AdV. Mater. 2003, 15, 1428.
(13) Terada, M.; Furuya, K.; Okamura, T.; Morimoto, I.; Nakao, M. Jpn.
J. Appl. Phys. 1993, 32, 5219.
(20) Sanchez-Juarez, A.; Ort´ız, A. Semicond. Sci. Technol. 2002, 17, 931.
10.1021/ic048276l CCC: $30.25
Published on Web 04/19/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 10, 2005 3755

Mitzi
Table 1. Crystallographic Data for (N2H5)4Ge2Se6 (1) and
(N2H4)3(N2H5)4Sn2Se6 (2)
because of the possibility of using low-temperature, high-
throughput (with potential for large area) processes such as
spin coating, drop casting, printing, or stamping. However,
the covalency of the extended metal chalcogenide framework
generally leads to low solubility in common solvents.
chemical formula
fw
(N2H5)4Ge2Se6
751.19
(N2H4)3(N2H5)4Sn2Se6
939.53
space group
a, Å
b, Å
I41cd (No. 110)
12.708(1)
12.708(1)
P1h (No. 2)
6.6475(6)
9.5474(9)
9.8830(10)
94.110(2)
99.429(2)
104.141(2)
595.9(2)
1
2.618
0.71 073 (Mo KR)
112.8
2.30
2.82
Recently, we reported the deposition of high-quality,
continuous, ultrathin (<10 unit cells thick) main-group metal
chalcogenide films from hydrazine-based solutions of SnS2
and SnSe2.7 The key to this process was the identification
of hydrazine as a solvent and the incorporation of excess
chalcogen (e.g., S or Se) in the solution, thereby enabling
the formation of suitable highly soluble hydrazinium-based
precursors [e.g., (N2H5)4Sn2S6 and (N2H4)3(N2H5)4Sn2Se6].
Thin films of the precursors may be deposited by spin coating
in air at room temperature, followed by a brief annealing
step at < 300 °C (under inert atmosphere) to effect
decomposition to the targeted metal chalcogenide film. The
resulting films, incorporated as channel layers within TFTs,
exhibit n-type field-effect mobilities >10 cm2/V sec, ap-
proximately 1 order of magnitude higher than the mobilities
of previous spin-coated channel layers (either n- or p-type).
The structure and thermal properties of the tin(IV) sulfide
precursor (N2H5)4Sn2S6 have been previously reported.7 Here,
we report the preparation, detailed structure, and thermal
properties of several analogous hydrazinium-based metal
selenide precursors based on tin(IV) and germanium(IV)
selenide anions.
c, Å
21.955(2)
R, deg
â, deg
γ, deg
V, Å3
Z
3545.6(5)
8
2.814
0.71 073 (Mo KR)
157.0
1.92
2.68
F
calcd, g/cm3
wavelength, Å
abs coeff (µ), cm-1
Rfa
b
Rw
a Rf ) Σ(|Fo| - |Fc|)/Σ(|Fo|). b Rw ) [Σw(|Fo| - |Fc|)2/Σ(w|Fo| )]1/2
.
2
(N2H4)3(N2H5)4Sn2Se6 (2). Yellow crystals of 2 are formed by
dissolving (in a nitrogen atmosphere, over several minutes, with
stirring) 0.277 g of SnSe2 (1 mmol) in 1 mL of hydrazine and 0.079
g of Se (1 mmol), yielding, ultimately, a light-yellow solution.
Evaporating the solution under flowing nitrogen gas over a period
of several hours leads to the formation of approximately 0.450 g
of a yellow crystalline powder (96% yield). The powder loses
weight while sitting on a balance at room temperature, reflecting a
loss of solvent (i.e., hydrazine) from within the structure. Chemical
analysis, observed: N(20.8%), H(3.5%). Calcd: N(20.9%), H(3.4%).
X-ray Crystallography. A pale-yellow blocklike crystal of
(N2H5)4Ge2Se6 (1) [(N2H4)3(N2H5)4Sn2Se6 (2)], with the approximate
dimensions 0.03 mm × 0.05 mm × 0.18 mm [0.11 mm × 0.12
mm × 0.46 mm], was selected under a microscope and attached to
the end of a quartz fiber with five-minute epoxy. Intensity data
were collected with a Bruker SMART CCD diffractometer equipped
with a fine focus 2.4 kW sealed tube X-ray source (Mo KR
radiation). The data were collected at room temperature using an
exposure time of 60 s [6 s] per frame. For 2, a relatively short
collection time was employed to minimize hydrazine loss from the
sample during data collection.
Final cell parameters (Table 1) and a crystal orientation matrix
were obtained by a least-squares fit of 4526 [3504] reflections. An
empirical absorption correction based on equivalent reflections was
applied to the intensity data.21 The structure was solved and refined
using the NRCVAX 386 PC version program.22 First, the Ge [Sn]
and Se atoms were located using direct methods. The N atoms were
then located using successive Fourier difference maps. All heavy
atoms (Ge, Sn, Se, and N) were refined anisotropically. Hydrogen
atoms were located from difference maps and fully refined using
isotropic thermal parameters. The minimum and maximum peaks
in the final difference Fourier map correspond to -0.4 and 0.5 e/Å3
[-0.6 and 0.6 e/Å3]. No additional symmetry was detected for the
refined structures using the MISSYM program.23 For 1, refinement
yielded a flack parameter of -0.02(8). Crystallographic results for
the two compounds are summarized in Table 1. Selected bond
lengths and angles for each of the compounds are given in Table
2. A complete listing of crystallographic data (in CIF format) is
given in the Supporting Information.
Experimental Section
Synthesis. (N2H5)4Ge2Se6 (1). A multiphase yellow product is
obtained by reacting (under an inert nitrogen atmosphere) 2 mmol
of GeSe2 (0.461 g) and 2 mmol of Se (0.158 g) in 1 mL of freshly
distilled hydrazine. NOTE: hydrazine is highly toxic and should
be handled using appropriate protective equipment to prevent
contact with either the vapors or the liquid. The reaction
produces vigorous bubbling and rapidly yields a clear light-yellow
solution. The solution is evaporated overnight to dryness under a
N2 flow and transferred into a drybox (where it is allowed to further
dry in the drybox antichamber for several hours), yielding ap-
proximately 753 mg of a bright-yellow crystalline product. Chemical
analysis yielded N(21.9%) and H(3.6%). Note that these values
are more consistent with those calculated for (N2H4)3(N2H5)4Ge2-
Se6 [N(23.1%) and H(3.8%)] than with those for (N2H5)4Ge2Se6
[N(14.9%) and H(2.7%)]. Indeed, powder X-ray diffraction (XRD)
indicates a mixture of peaks from (N2H5)4Ge2Se6 (1) (minor
component), as well as (more predominantly) from a phase with
similar lattice constants to that expected for (N2H4)3(N2H5)4Sn2Se6
(2). The diffraction pattern rapidly changes in air as the material
loses hydrazine, ultimately yielding a more single-phase pattern
consistent with that expected for 1. A crystal was isolated for the
hydrazine-rich phase, (N2H4)3(N2H5)4Ge2Se6, and the approximate
triclinic lattice constants were determined to be a ) 6.597(2) Å, b
) 9.381(3) Å, c ) 9.745(3) Å, R ) 93.67(1)°, â ) 100.34(1)°,
and γ ) 104.97(1)° (i.e., similar to those for the Sn compound
2ssee Table 1). Although a full data set was collected and solved
for (N2H4)3(N2H5)4Ge2Se6, confirming an isostructural relationship
to 2, the quality of this refinement was poor because of the rapid
loss of hydrazine from the sample during data collection. From
the mixed-phase sample, a single crystal of 1 was also selected for
a full structure analysis (see Table 1).
Powder X-ray Diffraction. XRD powder patterns were collected
for thermogravimetric analysis (TGA) products using a Siemens
(21) Sheldrick, G. M. SADABS; Institut fu¨r Anorganische Chemie der
Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(22) Gabe, E. J.; Le Page, Y.; Charland, J.-P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384.
(23) Le Page, Y. J. Appl. Crystallogr. 1988, 21, 983.
3756 Inorganic Chemistry, Vol. 44, No. 10, 2005

Soluble Hydrazinium Ge(IV) and Sn(IV) Selenide Salts
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1 and 2
chalcogen to the hydrazine to facilitate the formation of the
soluble precursor.
(N2H5)4Ge2Se6 (1)
(N2H4)3(N2H5)4Sn2Se6 (2)
The soluble hydrazinium MSe2 (M ) Ge or Sn) precursors
are isolated by stirring metal(IV) selenide and selenium in
hydrazine under ambient conditions. Dissolution primarily
occurs through the overall reaction:
Ge1-Se1
2.415(3)
2.415(3)
2.316(3)
2.316(3)
2.424(3)
2.424(3)
2.305(3)
2.305(3)
1.450(7)
1.43(2)
Sn-Se1
2.4609(5)
2.5933(5)
2.5909(4)
2.4639(5)
2.5909(4)
1.434(6)
1.439(6)
1.426(6)
1.462(8)
Ge1-Se1a
Sn-Se2
Ge1-Se3
Sn-Se2b
Ge1-Se3a
Sn-Se3
Ge2-Se1
Se2-Snb
Ge2-Se1a
N1-N2
4-
5N2H4 + 2Se + 2MSe2 f N2 + 4N2H5+ + M2Se6
(1)
Ge2-Se2
N3-N4
Ge2-Se2a
N5-N6
N1-N2
N7-N7c
The pale-yellow color of each solution suggests the presence
of at most a low concentration of polychalcogenide anions27
and vigorous bubbling during dissolution supports the gas
generation indicated in eq 1. Evaporation of the solution over
a 24 h period yields the hydrazinium precursor crystals. Thin
films of the precursor can similarly be prepared by spin
coating the solution onto a suitable substrate.7
N3-N4
Se1-Sn-Se2
Se1-Sn-Se2b
Se1-Sn-Se3
Se2-Sn-Se2b
Se2-Sn-Se3
Se2-Sn-Se3b
Sn-Se2-Snb
110.64(2)
Ge1-Se1-Ge2
Se1-Ge1-Se1a
Se1-Ge1-Se3
Se1-Ge1-Se3a
Se1a-Ge1-Se3a
Se3-Ge1-Se3a
Se1-Ge2-Se1a
Se1-Ge2-Se2
Se1-Ge2-Se2a
Se2-Ge2-Se2a
85.1(1)
95.2(2)
110.78(2)
116.36(2)
95.59(1)
110.40(2)
111.16(2)
84.41(1)
110.64(5)
110.82(5)
110.64(5)
116.7(2)
94.7(2)
110.55(5)
111.78(5)
115.6(2)
Although in our earlier studies of film deposition7 we
generated the hydrazinium precursor in situ during a spin-
coating process (from a hydrazine solution), the precursor
can also be isolated in crystalline form independent from
the spin-coating process (as done in the current study). This
enables the identification of other solvents for the precursor,
rather than relying on the highly toxic and explosive
hydrazine as a spin-coating solvent. We have recently shown,
for example, that an indium(III) selenide precursor can be
dissolved in an ethanolamine/dimethyl sulfoxide mixture.28
The solutions formed using alternative (non-hydrazine-based)
solvents may be useful for processing films, with the choice
of solvent governed by the nature of the substrate surface
(e.g., hydrophilic or hydrophobic) and the film composition/
grain morphology desired. Although 1 and 2 are highly
soluble in hydrazine, alternative solvents identified for these
systems include amine-based solvents such as butylamine,
ethylenediamine, and 3-methoxypropylamine, as well as
dimethyl sulfoxide. Precursor 1, for example, readily dis-
solves within a few minutes (at room temperature, with
stirring) to form a clear yellow solution in each of the amine-
based solvents listed above at a ∼0.5 M concentration (higher
concentrations are likely also possible, although this was not
addressed in the current study). DMSO solutions of 1 have
also been prepared at the ∼0.1 M concentration level
(dissolution takes several hours with stirring at room tem-
perature). The resulting solutions appear to be stable for at
least several days (longer time periods were not considered
in this study) under inert atmosphere conditions.
a 1 - x, -y, z. b 1 - x, 1 - y, 1 - z. c -x, -y, -z.
D5000 diffractometer (Cu KR radiation). The specimen for analysis
was prepared by pressing powder of the sample into vacuum grease
on a glass slide. No attempt was made to control preferred-
orientation effects. The phases present were identified (e.g., SnSe2,
GeSe2, Se) by comparison with powder diffraction file (PDF) cards
or, in the case of the new compound (N2H5)4Sn2Se6, by comparison
with the calculated powder pattern for the analogous compound
(N2H5)4Ge2Se6 (1). In cases where a refinement of lattice constants
was performed from the diffraction data [for GeSe2, see Figure 7b;
for (N2H5)4Sn2Se6, see Figure 6a], initial cell constants from the
PDF card (for GeSe2) or from 1 [for (N2H5)4Sn2Se6] were used. A
Pawley refinement24 was used to optimize the pattern parameters
using the Accelrys MS Modeling package. The peak profiles were
fitted using a pseudo-Voigt function. The final Pawley refinement
Rwp values were 8.05% for the (N2H5)4Sn2Se6 data and 5.52% for
the GeSe2 data.
Thermal Analysis. TGA scans were performed in a flowing
nitrogen atmosphere at 2 °C/min using a TA Instruments TGA-
2950 system.
Results and Discussion
Solution Processibility. While the solvent properties of
hydrazine have long been of interest,25 little solubility has
been noted for the metal chalcogenides (although sulfur itself
was reported to be highly soluble). Polysulfide- and poly-
selenide-based hydrazine hydrate solutions have, in fact, been
used to precipitate transition and main-group metal chalco-
genides,26 using the corresponding solutions of the metal salts
to effect the precipitation. In the current study, hydrazine is
being used as a solvent for the metal chalcogenide systems,
rather than as a precipitation medium. The differences come
in the metal chalcogenides being considered (i.e., primarily
main-group metal systems), the relatively concentrated
hydrazine solvent employed, and the addition of extra
Crystal Structures. Each of the title metal chalcogenide
precursors consist of discrete M2Se64- (M ) Sn, Ge) anions
of edge-sharing MSe4 tetrahedra, as previously found for
(N2H5)4Sn2S6.7 For (N2H5)4Ge2Se6 (1) (Figure 1), the Ge2Se64-
anions have longer Ge-Se bridging [2.415(3)-2.424(3) Å]
bonds than terminal [2.305(3)-2.316(3) Å] bonds and have
Se-Ge-Se bond angles ranging from 94.7(2)° to 116.7(2)°
(Table 2). These bond lengths and angles are in good
agreement with the corresponding values observed for the
4-
Ge2Se6 moiety in Na4Ge2Se6‚16H2O.29
(24) Pawley, G. S. J. Appl. Crystllogr. 1981, 14, 357.
(25) Welsh, T. W. B.; Broderson, H. J. J. Am. Chem. Soc. 1915, 37, 816.
(26) (a) Davies, D. A.; Vecht, A.; Silver, J.; Marsh, P. J.; Rose, J. A. J.
Electrochem. Soc. 2000, 147, 765. (b) Marsh, P. J.; Davies, D. A.;
Silver, J.; Smith, D. W.; Withnall, R.; Vecht, A. J. Electrochem. Soc.
2001, 148, D89.
(27) Bonnaterre, R.; Cauquis, G. J. Chem. Soc., Chem. Commun. 1972,
293.
(28) Mitzi, D. B.; Copel, M.; Chey, S. J. AdV. Mater. 2005, in press.
(29) Krebs, B.; Mu¨ller, H. Z. Anorg. Allg. Chem. 1983, 496, 47.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3757

Mitzi
Figure 1. (a) Crystal structure of (N2H5)4Ge2Se6 (1), viewed down the
[010] direction (shown in polyhedral representation). Dashed lines indicate
the unit cell outline. For clarity, atoms are represented as spheres, with
uniform sizes selected for each atom type. Within the hydrazinium cations,
+
the NH3 nitrogen is depicted as a red-filled sphere, whereas the NH2
nitrogen is depicted as an unfilled sphere. Hydrogen atoms are shown as
small gray-filled spheres. (b) The detailed structure of the Ge2Se64- anion
and NH2NH3+ cations (with atom labeling shown). For clarity, the hydrogen
atoms have been removed. The thermal ellipsoids for Ge, Se, and N atoms
are drawn at 50% probability.
Figure 2. Parts a and b: Two successive layers (ab plane) progressing
along [001], cut from the crystal structure for 1 (see Figure 1a). The
Ge2Se64- anion is shown in polyhedral representation. Dashed lines indicate
the unit cell outline. Dotted lines indicate hydrogen bonding between
+
neighboring hydrazinium cations. Within the hydrazinium cations, the NH3
4-
The isolated Ge2Se6 anions are charge balanced by a
nitrogen is depicted as a red-filled sphere, whereas the NH2 nitrogen is
depicted as an unfilled sphere. Hydrogen atoms are shown as small gray-
filled spheres.
network of hydrazinium cations. The cations form extended
hydrogen-bonded chains of repeating -N3‚‚‚H-N2-
N1‚‚‚H-N4-N3‚‚‚ lengths, which extend alternatively
(progressing down the c axis) along the [100] or the [010]
shown in Figure 2) and Ge2Se64- anions in an adjacent layer,
thereby binding the layers together into a more three-
dimensional structure.
4-
direction and weave among the Ge2Se6 anions to form
charge-balanced layers (Figure 2). Analogous hydrogen-
bonded chains have been observed in (N2H5)Cl, (N2H5)-
HC2O4, and (N2H5)H2PO4,30-32 with hydrogen bonding
For (N2H4)3(N2H5)4Sn2Se6 (2), the structure (Figure 3)
consists of dimers of edge-sharing tin(IV) selenide tetrahedra,
similar to the germanium analogue, with Sn-Se bond lengths
of 2.4609(5)/2.4639(5) (Sn-Se1/Sn-Se3) Å for the two
terminal bonds and 2.5909(4)/2.5933(5) Å for the two
bridging Sn-Se2 bonds (Table 2). The observed Sn-Se
distances, as well as the intradimer Sn‚‚‚Sn interaction
[3.4827(5) Å], are in good agreement with other compounds
+
extending from the NH3 group of one cation to the NH2
group of a neighboring cation. The hydrogen-bonding
distances (distances between nitrogens) for 1, N3‚‚‚H-N2
[2.84(1) Å] and N1‚‚‚H-N4 [2.85(1) Å], are relatively short
compared to the previously reported values for the reported
hydrazinium-based compounds (2.87-3.14 Å).30-32 In ad-
dition to the relatively strong hydrogen bonding among the
hydrazinium cations, significant interaction between selenium
4-
4-
comprised of Sn2Se6 anions.34,35 The Sn2Se6 anions are
separated by both neutral hydrazine molecules and mono-
protonated hydrazinium cations. The structure (Figure 3a)
can nominally be considered as comprised of layers of
Sn2Se64- anions (extending in the ac plane), sandwiched on
both sides by layers of hydrazinium cations. Incorporated
between the electrically neutral cation-anion-cation sand-
wich structures are single layers of neutral hydrazine
molecules. The N-N distances are typical for hydrazine
species, with slightly shorter (though the difference is only
marginally statistically significant) average bond distances
+
atoms and the NH3 groups of the hydrazinium cations is
also noted, with short Se‚‚‚H-N distances (Se to N) that
include Se3‚‚‚H-N2 [3.31(1) Å], Se3‚‚‚H-N4 [3.35(1) Å],
and Se2‚‚‚H-N2 [3.32(1) Å]. The Se‚‚‚H-N angles for these
contacts fall in the range 161-172° (reasonable for hydrogen-
bonding interactions), and Se-to-N distances are all shorter
than those observed for Se‚‚‚H-N hydrogen bonding in
[Mn(C2H8N2)3]2Sb2Se5 and (H3NC2H4NH2)3SbSe4 (3.40-
3.67 Å).33 These short selenium-nitrogen interactions are
principally between hydrazinium cations in one layer (as
(33) (a) Na¨ther, C.; Wendland, F.; Bensch, W. Acta Crystallogr., Sect E
2003, 59, m119. (b) Wendland, F.; Na¨ther, C.; Schur, M.; Bensch,
W. Z. Naturforsch. 1998, 53b, 1144.
(34) Eisenmann, B.; Hansa, J. Z. Kristallogr. 1993, 203, 299.
(35) Krebs, B.; Uhlen, H. Z. Anorg. Allg. Chem. 1987, 549, 35.
(30) Sakurai, K.; Tomiie, Y. Acta Crystallogr. 1952, 5, 293.
(31) Nilsson, A.; Liminga, R.; Olovsson, I. Acta Chem. Scand. 1968, 22,
719.
(32) Liminga, R. Acta Chem. Scand. 1965, 19, 1629.
3758 Inorganic Chemistry, Vol. 44, No. 10, 2005

Soluble Hydrazinium Ge(IV) and Sn(IV) Selenide Salts
Figure 4. Two views of the hydrogen bonding (dotted lines) among
hydrazinium cations in 2 (a) viewed along [100] (as in Figure 3) and (b)
viewed approximately along [010]. Within the hydrazinium cations, the
NH3+ nitrogen is depicted as a red-filled sphere, whereas the NH2 nitrogen
is depicted as an unfilled sphere. Hydrogen atoms are shown as small gray-
filled spheres. In part b, a single hydrogen-bonded heptamer of hydrazinium
cations is highlighted.
Figure 3. (a) Crystal structure of (N2H4)3(N2H5)4Sn2Se6 (2), viewed down
the [100] direction (shown in polyhedral representation). Dashed lines
indicate the unit cell outline. For clarity, atoms are represented as spheres,
with uniform sizes selected for each atom type. Within the hydrazinium
+
cations, the NH3 nitrogen is depicted as a red-filled sphere, whereas the
NH2 nitrogen is depicted as an unfilled sphere. Hydrogen atoms are shown
as small gray-filled spheres. The dotted lines indicate short Se1‚‚‚Se1
interactions between layers of anions. The hydrazinium cations to be
depicted in Figure 4 are highlighted (darker lines). (b) The detailed structure
4-
+
of the Sn2Se6 anion, the NH2NH2 molecules, and the NH2NH3 cations
(with atom labeling shown). For clarity, the hydrogen atoms have been
removed. The thermal ellipsoids for Sn, Se, and N atoms are drawn at 50%
probability.
for the cationic species [1.426(6) and 1.434(6) Å] compared
to those for the neutral molecule [1.439(6) and 1.462(8) Å].
The neutral hydrazine molecules are held within the
structure by hydrogen bonding to adjacent hydrazinium
cations (Figure 4). Alternating hydrazinium cations and
hydrazine molecules form hydrogen-bonded heptamers, N6-
N5‚‚‚H-N4-N3‚‚‚H-N1-H‚‚‚N7-N7‚‚‚H-N1-H‚‚‚N3-
N4-H‚‚‚N5-N6, which form short, twisted chains among
the Sn2Se64- anions (Figure 4). The relatively short hydrogen-
bonding distances (distances between nitrogens) for 2sN5-
H‚‚‚N4 [2.87(1) Å], N3‚‚‚H-N1 [2.81(1) Å], and N1-
H‚‚‚N7 [2.81(1) Å]sare similar to the values observed in
compound 1. In addition to hydrogen bonding among the
hydrazinium cations, there are also significant hydrogen-
Figure 5. (a) TGA of the precursors (N2H5)4Sn2S6 (dashed black line;
from ref 7) and 2 (solid red line; 2 °C/min ramp rate, flowing nitrogen).
The arrows mark the points along the decomposition pathway for which
an XRD pattern was taken (see Figure 6). (b) TGA scans for sulfur and
selenium, run under the same conditions as for the hydrazinium precursors.
+
bonding interactions between selenium atoms and the NH3
solution deposition of metal chalcogenide semiconductors,
is the low temperature at which the precursors decompose.
For (N2H5)4Sn2S6, decomposition in an inert nitrogen atmo-
sphere is essentially completed by 200 °C (Figure 5a).7 The
observed 34.6% weight loss (at 300 °C) is in good agreement
with the expected value (34.9%) for the formation of SnS2
(through the loss of four hydrazines and two hydrogen
sulfides or their decomposition products). Decomposition of
the analogous selenium-based precursor, 2, occurs in several
steps (Figure 5a). The first transition begins as early as room
temperature and is complete by approximately 90 °C, with
the observed weight change (-8.7%) in agreement with the
loss of three hydrazines from 2 (-10.2% expected). The
groups of the hydrazinium cations, with Se‚‚‚H-N distances
(Se to N) that include Se1‚‚‚H-N5 [3.38(1) Å], Se3‚‚‚H-
N1 [3.45(1) Å], and Se3‚‚‚H-N5 [3.44(1) Å]. These values
are slightly longer than the corresponding distances obtained
for 1. A short Se1‚‚‚Se1 interaction [3.493(1) Å] is also
4-
present among Sn2Se6 moieties (Figure 3). This distance
is significantly less than twice the ionic36 or van der Waals37
radius for Se (∼3.8 Å).
Thermal Analysis. An important feature of the hy-
drazinium-based precursors, with respect to their use in the
(36) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
(37) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Inorganic Chemistry, Vol. 44, No. 10, 2005 3759

Mitzi
Figure 7. (a) TGA of the GeSe2 precursor (2 °C/min ramp rate, flowing
nitrogen). The arrows mark the points along the decomposition pathway
for which an XRD pattern was taken (275 and 550 °C). (b) Powder XRD
patterns (Cu KR radiation) for the GeSe2 hydrazinium precursor after heating
to 275 and 550 °C (using the same conditions as those for the TGA scans).
The diffraction pattern in part b (550 °C curve) matches that of GeSe2 (PDF
30-0595). A Pawley refinement of the parameters for this pattern yielded
a monoclinic cell with a ) 7.0126(2) Å, b ) 16.8341(6) Å, c )
11.8297(6) Å, and â ) 90.729(1)°.
Figure 6. Powder XRD patterns (Cu KR radiation) for precursor 2 after
heating to (a) 90 °C and (b) 215 or 425 °C (using the same conditions as
those for the TGA scans, see Figure 5a). Indexing of the powder pattern
for the (N2H5)4Sn2Se6 product in part a is given in the Supporting
Information (i.e., h, k, l, d spacing, normalized intensities), as determined
by Pawley refinement. The reflection indices given above the 425 °C data
in part b are those for the SnSe2 structure.40 The asterisks in the 215 °C
data indicate the reflection positions for elemental Se (PDF 06-0362).
resulting product is isostructural with 1 (Figure 6a), with
refined tetragonal lattice constants a ) 12.775(1) Å and c
) 22.705(1) Å. The slightly larger cell volume for (N2H5)4-
Sn2Se6 compared with that of (N2H5)4Ge2Se6 (1) (3705.5
versus 3545.6 Å3) is consistent with the larger atomic radius
of Sn. The chemical analysis of the product is also in good
agreement with that expected for (N2H5)4Sn2Se6sexpected
(N: 13.29%, H: 2.39%), found (N: 13.3%, H: 2.3%). The
second transition, complete by approximately 200 °C,
corresponds to a total weight change of approximately
-24.2%, in good agreement with the weight change expected
for the loss of four more hydrazine molecules from 2
(-23.9%, including the weight loss from the first transition).
As for the decomposition of the sulfide system (N2H5)4Sn2S6,7
the transition to crystalline SnSe2 is completed by ∼200 °C
(i.e., see the XRD pattern in Figure 6b, 215 °C curve). In
contrast to (N2H5)4Sn2S6, however, for which the hydrazinium
cation decomposes/dissociates from the sample at essentially
the same temperature as the corresponding sulfide anion
(presumably as hydrazine and hydrogen sulfide or their
decomposition products), in 2 selenium remains in the sample
after the loss of the hydrazinium species, as is evident from
the XRD pattern in Figure 6b (215 °C curve). Note that
hydrogen chalcogenides can decompose to hydrogen and
chalcogen at moderate temperatures,38 and metal chalco-
genides may act to catalyze this reaction.39 For 2, the third
major weight loss transition, which is complete by ap-
proximately 350 °C (yielding a total loss of -40.9% by 400
°C), corresponds to the loss of the two seleniums to yield
SnSe2 (total expected weight loss for the three transition ∼
-41.1%). As indicated in Figure 6b (425 °C curve), the
resulting product is phase-pure crystalline SnSe2 (CdI2
structure type).40 The lower volatility of selenium relative
to sulfur (Figure 5b) implies that a higher temperature is
required to achieve a clean sample of the targeted semicon-
ductor (i.e., to lose the extra elemental selenium) relative to
the decomposition of (N2H5)4Sn2S6.
Decomposition of the germanium selenide precursor
(Figure 7) is complicated by the fact that the as-prepared
bulk precursor is not single phase but, rather, a mixture of 1
with a majority hydrazine-rich phase [i.e., (N2H4)3(N2H5)4-
Ge2Se6, a compound isostructural to 2], as described in the
Experimental Section. Furthermore, in contrast to the thermal
decomposition of 2, for which there is a small temperature
region over which the hydrazine-depleted phase (N2H5)4Sn2-
Se6 is stable, for the mixture of 1 and (N2H4)3(N2H5)4Ge2-
Se6, no significant plateau corresponding to 1 appears in the
weight loss versus temperature data, making it difficult to
prepare a single-phase sample of 1 via thermal decomposition
(at least under the conditions examined). The decomposition
pathway is also different in the sense that, although the
decomposition of hydrazine from the sample appears to be
complete by approximately 200 °C (as for 2), the product
obtained upon this decomposition appears to be amorphous
(Figure 7b, 275 °C curve). Furthermore, the excess selenium
(38) Pearson, R. K.; Haugen, G. R. Int. J. Hydrogen Energy 1981, 6, 509.
(39) Chivers, T.; Lau, C. Int. J. Hydrogen Energy 1987, 12, 235.
(40) Busch, G.; Fro¨hlich, C.; Hulliger, F.; Steigmeier, E. HelV. Phys. Acta
1961, 34, 359.
3760 Inorganic Chemistry, Vol. 44, No. 10, 2005

Soluble Hydrazinium Ge(IV) and Sn(IV) Selenide Salts
that must remain in the sample at this stage of decomposition
proves much more difficult to remove from the sample than
for 2 and requires treatment at higher temperatures. Never-
theless, at temperatures above 500 °C, a single-phase product
of GeSe2 is achieved (Figure 7b, 550 °C curve). The total
weight loss at 550 °C, 45.3%, is approximately what would
be expected for the decomposition of (N2H4)3(N2H5)4Ge2-
Se6 to GeSe2, consistent with the observation from the
chemical analysis and powder XRD that this is the majority
phase in the as-prepared germanium selenide precursor. Note
that at temperatures above 550 °C, the GeSe2 product
sublimes. Therefore, a small fraction of the observed weight
loss at 550 °C may already be attributed to this sublimation.
Given the higher temperatures required for decomposition,
the hydrazinium-based GeSe2 precursor appears to be less
well-suited for use as a convenient spin-coatable metal
chalcogenide precursor (compared with the SnS2 and SnSe2
systems).
metal chalcogenide films. In fact, for the tin(IV) chalcogenide
systems, high-quality films have already been demonstrated
and used as channel layers for TFTs.7
In a more general sense, the proposed solution-processing
technique for metal chalcogenides7 relies on the formation
of a precursor that (1) has the extended metal chalcogenide
framework broken up into smaller inorganic anions (enhanc-
ing solubility) and (2) has a countercation that is volatile
and small enough for low-temperature removal from the film
with minimal disruption (thereby enabling convenient re-
covery of the extended semiconducting framework after spin
coating). It, therefore, seems likely that the hydrazinium
precursor approach will be extendable beyond the hy-
drazinium-based germanium(IV) and tin(IV) systems de-
scribed here, as well as to a wider range of solvent systems.
We have recently demonstrated, for example, that high-
quality In2Se3 films can be deposited using an indium
selenide-based hydrazinium precursor dissolved in an etha-
nolamine/dimethyl sulfoxide (DMSO) mixed solvent.28 The
resulting high-quality In2Se3 films have proven useful as TFT
channel layers, yielding mobilities as high as 16 cm2/V sec.
Furthermore, preliminary results have also yielded Sb2S3 and
Sb2Se3 films,7 spin coated from hydrazine-based solutions.
So far, the facile formation and characterization of soluble
hydrazinium-based precursors has been primarily limited to
main-group metal chalcogenides. Corresponding initial at-
tempts with several transition-metal-based chalcogenides
(e.g., Cu2Se, CdSe) have not been successful, as a result of
the limited solubility of these systems in the hydrazine
solvent under analogous conditions to those employed for
the main-group metal systems.
Conclusion
Two new hydrazinium-based germanium(IV) (1) and tin-
(IV) (2) selenide systems have been discussed. Each structure
is found to consist of very similar M2Se64- (M ) Ge or Sn)
anions interspersed with hydrazinium cations and, in the case
of 2, additional neutral hydrazine molecules. Each is
stabilized by a range of bonding interactions, including
relatively strong hydrogen bonding among the hydrazinium
cations and hydrazine molecules, as well as among the
4-
hydrazinium cations and the M2Se6 anions. Additionally,
for compound 2, a short Se‚‚‚Se distance among anions
indicates a further stabilizing interaction. The hydrazinium
cations (and hydrazine molecules) dissociate from the
structure upon heating in an inert atmosphere by ap-
proximately 200 °C. For the selenides, 1 and 2, however,
temperatures > 300 °C (especially for the Ge-based 1) are
required to achieve a phase-pure crystalline metal chalco-
genide product [in contrast to the situation for (N2H5)4Sn2S6],
as a result of the higher temperature required to dissociate
the excess selenium. Each of the ionic structures is highly
soluble in hydrazine, as well as a range of other solvents,
including amines and DMSO. As such, they are potentially
useful as precursors for spin coating or solution processing
Acknowledgment. The author thanks A. Afzali for
distilling the hydrazine used in this study.
Supporting Information Available: An X-ray crystallographic
file, in CIF format, containing information for (N2H4)3(N2H5)4Sn2-
Se6 and (N2H5)4Ge2Se6. Table of observed XRD peaks for (N2H5)4-
Sn2Se6, including indexing and normalized peak intensities. This
material is available free of charge via the Internet at
http://pubs.acs.org.
IC048276L
Inorganic Chemistry, Vol. 44, No. 10, 2005 3761